The United States Government has a world wide, non-exclusive, non-transferrable, irrevocable, paid-up license to practice or have practiced, for or on behalf of the United States, the subject invention.
This invention relates to high speed wireless modems and more particularly to a high data rate modem utilizing fast frequency hopping for each symbol and phase converted to a frequency offset.
There is an increasing need to increase data transmission capacity to support rapid dissemination of imagery, video and status information for all types of wireless communication. Although satellite and microwave systems can provide some of this capacity, there is a continuing need to provide connectivity among mobile units.
As discussed by D. L. Herrick and P. K. Lee in an article entitled xe2x80x9cChess: A New Reliable High Speed, HF Radioxe2x80x9d, MILCOM ""96, McLean, Va., October 1996, current military HF radio supports raw data transmission at 4,800 bps. However, with error correction for reliable transmission of data this rate drops to around 2,400 bps or even lower. With the increasing reliance on digital transmission, there is a need to increase the data rate while maintaining current transmission ranges.
Also, because the available bandwidth is limited, the new waveform must have increased spectral efficiency while maintaining compatibility with existing radios. For instance, existing VHF and UHF radios operate on a 25 KHz channel spacing, either single-channel or with a fixed hop rate. For compatibility, it is desired to maintain the same instantaneous bandwidth and a compatible or non-interfering hopping pattern.
In addition to the noise and multipath that are common to both commercial and military systems, the military must also deal with intentional jamming. Published work on advanced modulations and codes generally assumes slow frequency hopping and noncoherent demodulation. Although there was some published work on fast frequency hopping and coherent demodulation, work is limited to performance analysis of an assumed fast frequency hopping and coherent demodulation. No specific techniques have been revealed.
One system for providing coherent communications in a non-coherent frequency hopping system is described in U.S. Pat. No. 5,150,378 in which a known bit sequence is used to achieve coherent demodulation in a slow frequency-hopped system. Note that coherent demodulation refers to the detecting of both amplitude and phase. As described in the patents, in this frequency-hopped system, many symbols are transmitted for each frequency hop. Note that the slow frequency hopping described in this patent permits coherent demodulation. However, this system cannot support fast frequency hop rates. In fast frequency hop, only one symbol is transmitted for each hop. Moreover, the BPSK system described in this patent is not spectrum efficient.
In summary, present wireless modems to date permit only low to medium data rates and then with poor spectrum utilization efficiency, typically equal to or less than 1 bit/s/Hz. Moreover, these modems only support slow frequency hopping. Slow frequency hopping supports either non-coherent demodulation which results in poor performance or repeater jamming; or coherent demodulation requires additional reference bits to improve bit error rates, as discussed in the above patent. In either case, slow frequency hopping is vulnerable to follower or repeater jamming.
In order to solve the above problems with prior wireless modems, the subject system utilizes a new technique called discrete trellis coded modulation and frequency hops individual symbols to permit fast frequency hopping which permits high data rates with efficient spectrum utilization of 3 to 5 bits/s/Hz. In order to improve the performance of fast frequency hopping, in the subject invention the phase of each symbol is converted to a frequency offset of each frequency hopped symbol. This permits coherent demodulation in which amplitude is detected normally and in which phase is detected in terms of the detected frequency offset. This permits improved error rate performance in the demodulation sector when using fast frequency hopping.
Note that for purposes of the subject invention, data rate performance is measured at bits per second; and spectrum utilization efficiency is measured at bits/second per Hz. Improved performance is defined in terms of bit error rate (BER) versus signal to noise ration (S/N) and in the presence of interference or jamming, measured at dB gain.
Advantages of the subject wireless modem includes high density modulation and coherent demodulation, coupled with fast frequency hopping that results in high spectrum utilization, high data rates, and repeater jamming mitigation.
More particularly, the subject modem employs a new high density signal design called discrete trellis-coded modulation (d-TCM) in which frequency hopping is applied to each symbol of the d-TCM. The encoding also includes applying a frequency offset to each symbol corresponding to its phase. Coherent signal demodulation involving detecting amplitude and frequency offset is employed to achieve strong performance.
The coherent signal demodulation takes advantage of the encoding of the phase of a symbol in terms of a corresponding frequency offset from an apparent carrier frequency to make detection of the phase more robust at the receive side.
The architecture of the d-TCM is based on the quadrature amplitude modulation (QAM) together with trellis coding that forms the conventional TCM. A text on quadrature amplitude modulation is authored by W. Webb and L. Hanzo, entitled xe2x80x9cModern Quadrature Amplitude Modulationxe2x80x9d, London, UK, Pentech Press, 1994. The distinction between d-TCM and TCM is that each symbol of the former is transmitted at a carrier frequency determined by a controlled key system, with the phase of each symbol being represented by a corresponding frequency offset. On the other hand, conventional trellis-coded modulation is transmitted at a fixed carrier frequency.
As commonly known, conventional TCM waveforms offer spectrum efficiency with attendant potential for high data rates. But, a conventional TCM waveform dwells on a fixed frequency, offering no anti jamming protection. At best, it offers slow frequency hopping that is vulnerable to repeater jamming.
The waveform used by the subject system avoids all the above problems by hopping each TCM symbol over a selected set of discrete frequencies, in one embodiment, with each frequency occupying a predetermined bandwidth, 3 to 6 kHz for HF communications. The bandwidth of 3 or 6 kHz is considered to be consistent with traditional HF channelization. At higher frequency bands, the channel bandwidth can increase to 25 kHz as commonly used for military VHF and UHF radios. In one embodiment, each frequency hopping pattern is controlled by a predetermined pseudo-random (PN) sequence to assure orthogonality of hopping patterns to permit multiple users to share the same frequency band. For a single 64 d-TCM channel, for example, the waveform can achieve raw data rate at 36 kbps at HF. 25 kbps or higher user data rates are achievable after two layers of error correction coding and their associated-interleavers. Powerful error correction codes are included to ensure quality communications. In one embodiment, two layers of error correction coding are included: a (6,5) trellis coding with Viterbi decoder with constraint length equal to seven (K=7) and a (13, 11) Reed-Solomon code. A channel interleaver is included to randomize channel errors.
In the above-described embodiment, much higher data rates are achievable by employing orthogonal frequency hopsets or a higher order d-TCM such as 128 or 256 d-TCM. The anti-jamming capability is derived from a wide bandwidth, 2 MHz for use in HF as an example, that the signals hop over. Through environment sensing, one embodiment of the subject wireless modem employs only those channels that are free of interference for its frequencies to hop on.
In one embodiment, to achieve reliable communications, a training sequence is used to assist signal acquisition and synchronization in which the training sequence is used to calibrate the portion of the symbol amplitude.
As part of the subject invention, coherent signal demodulation is used. Without the coherent demodulation, the system performance is expected to degrade up to 6 dB as compared with an optimal demodulation. A d-TCM symbol contains information in both the signal""s amplitude and phase. Signal amplitudes are measured and calibrated with the known preamble signals, thus recovering the information contained in the amplitude. On the other hand, effective encoding and decoding of the phase information does not presently exist, with prior art systems suffering a 6 dB degradation.
In the subject coherent demodulation scheme, phase information in a TCM symbol is encoded by a simple algorithm in which each phase for a symbol is translated into a predetermined frequency offset for each hop channel. For example, one has four possible phases in the d-TCM. Within a 2 MHz HF bandwidth, one has a total of about 333 6-kHz channels, among them, 200 channels are available. If the instantaneous frequency called for by the key stream is f0, for example, by the present algorithm, one can encode phase 1 as an f1, phase 2 as an f2, etc. Here, fn=f0xc2x1NxWz, N=1,2,3,4, and Wz is the instantaneous channel bandwidth, 6 kHz in this example. In principle, half of the instantaneous bandwidth is adequate. NxWz is the frequency offset from an apparent carrier frequency, f0, controlled by a key stream. Thus, if the data stream calls for phase 2 to be encoded, a 2xWz frequency offset will be added to or substrated from the carrier frequency such that the apparent carrier frequency plus or minus the 2Wz offset, will actually be transmitted. In the receiver, the search for f2 will follow with a narrow window search.
The subject system thus employs a new high density modulation waveform involving discrete trellis-coded modulation (d-TCM), and fast frequency hopping, 3 to 25 k hops/s depending on the radio""s channel bandwidth allocation, with coherent demodulation to achieve both high spectrum efficiency and high performance, frequency reuse and anti-jamming properties. All these features are desirable for the future military or commercial communications, but are unachievable in a single system with current wireless communications technologies.
Note that the subject wireless modem can also be used in other applications and frequency bands where efficient spectrum utilization, high data rate, high performance and immunity to interference and jamming are desired.
In summary, a system for increasing data transmission capacity over a wireless mobile link while at the same time providing improved jamming resistance utilizes discrete trellis-coded modulation involving frequency hopping individual symbols, along with encoding the phase of a symbol as a frequency offset to the hopped carrier frequency. This modulation system permits coherent demodulation in which both phase and amplitude of a symbol is robustly decoded and in which each phase of a quadrature amplitude modulated signal is decoded by the detected frequency offset, thus to provide reliable recovery of phase in the demodulation section. The resulting system improves spectrum efficiency and permits fast frequency hopping for improved jamming resistance, with the utilization of frequency offset coding permitting the coherent demodulation that improves the error-rate without the introduction of additional reference bits.